Integrated semiconductor devices are typically constructed in masse on a wafer of silicon or gallium arsenide. Each device generally takes the form of an integrated circuit (IC) die, which is bonded to the die-mounting paddle of a leadframe. The wire attachment pads on the die are connected with their corresponding leads on the leadframe with aluminum or gold wire during a wire bonding process. The die and leadframe are then encapsulated in a plastic or ceramic package, which is then recognizable as an IC "chip". IC chips come in a variety of forms such as dynamic random access memory (DRAM) chips, static random access memory (SRAM) chips, read only memory (ROM) chips, gate arrays, and so forth. The leads of each chip are connected to conductive traces by any number of techniques such as welding (for high-temperature applications), socketing or soldering. The conductive traces, which are formed by photolithography and etching processes, interconnect the chips and other discrete components such as resistors and capacitors. The traces are relatively delicate and can be easily damaged by improper handling of the boards.
The most commonly-used technique for soldering components and sockets to a printed circuit board is the soldered through-hole technique. With this technique, components are mounted on the top surface of a circuit board, with the leads of the components extending through metal-plated, trace-connected through-holes in the board which are of slightly larger diameter than the leads. The board is then subjected to a wave-soldering process. During the wave soldering process, solder is drawn by capillary action into the clearances between the leads and the through-hole walls. When the solder is allowed to solidify, the leads are securely soldered within the through-holes.
Following the soldering of the component leads within the through-holes, the lead extensions beneath the circuit board must be trimmed to a uniform, optimum length. This is typically done on a lead-trimming machine which operates much like a milling machine. However, since the lead trimmer's cutting head operates in a plane, uniformity of lead length is highly dependent on maintaining the circuit board in a plane that is parallel to that in which the cutting head operates as the leads are trimmed. Failure to maintain the parallel planarity will, at best, result in trimmed leads of varying thickness and, at worst, destruction of the circuit board by running the cutter head into the fiberglass material from which the board is constructed and destroying one or more of the conductive traces that interconnect the board's components.
Typically, circuit boards have been supported during the lead trimming process by support posts having a flat base with can be adhesively attached to the flat bed of the lead trimming machine so that they contact the surface on the component side of the circuit board at points where there are no components, no traces to damage, and no elevated solder regions that would cause the circuit board to warp slightly as it is supported. Each of the support posts must be individually located on the bed of the trimming machine by precise measurement of the circuit board. The edges of the circuit board are then supported by an adjustable frame having the same height as each of the support posts. When significant gaps have been sealed, a vacuum is applied to the lower surface of the board and it is held firmly in place against the support posts and adjustable frame. This support technique has at least four significant drawbacks. The first is the inordinate amount of time required for setup each time the leads of a different board must be trimmed; the second is the inconvenience of having to use an adhesive material to anchor the support posts to the lead trimming machine bed; the third is the high probability or erring when making measurements for support post placement, accompanied by a high probability of board damage before the error is discovered; and the fourth is the inherent instability of the support mechanism during the trimming operation.
What is needed is a more stable printed circuit board support apparatus that can be easily and rapidly adapted for a variety of different circuit boards, with much greater accuracy and, hence, a greatly minimized chance for the destruction of circuit boards.